1. Field of the Invention
This invention relates to a display device utilizing an element which exhibits the so-called electrochromic phenomenon.
Research and development of such electrochromic elements have recently been fluorishing owing to variety in the construction, the properties, for example; the persistence or memorizability thereof from the color formation (discoloration) to the disappearance of the color, availability of various materials for such element, the formed color tone of which changes in correspond to variations in applied voltage values, and so forth.
Typical examples of their application are electronic desk calculators, horological instruments and the like in which it is used as the element for indicating numerals, characters and symbols; a sign board, a meteorological chart board, a traffic sign board, an X-ray image display board and the like in which it is used as the element for displaying images; a soft facsimile display body capable of being repeatedly used by erasure as a light shutter or a facsimile signal recorder; and a writing board capable of being both inscribed and erased like an ordinary black board.
2. Description of the Prior Art
What is meant by the term "electrochromic phenomenon" is: a reversible coloring and decoloring phenomenon depending on electric polarity. The coloring (or discoloring) takes place by (1) application of electricity and the decoloring occurs by application of electricity having a polarity opposite to that of the electric current applied at the time of the abovementioned coloring or color formation, by (2) application of heat or by (3) the application of both electricity and heat. In an actual display element, however, it is possible to cause the color formation without depending on the apparent electric polarity.
The mechanism for the electrochromic phenomenon is not always simple and single. As one of several views heretofore made, the phenomenon is understood to be due to the so-called oxidation reduction reaction between an electrolyte and a color forming substance. In this case, the electrolyte and the color forming substance are not necessarily different from each other in the constituent material. In fact, there is a case in which the same substance can be the color forming substance simultaneously with the electrolyte. Also, from another aspect, there is such a view that the phenomenon takes place by variation in the light absorbing characteristic of the substance due to injection of electrons into the center of the color as is the case with photochromy. In practice, it is the result of the combination of these occurrences which is understood to be the cause of the electrochromic phenomenon.
Since the electrochromic phenomenon electrically changes a color which is possessed inherently by the material, the combination of possible colors is innumerable. Further, whether or not the material is capable of transmitting light, or whether or not it is capable of reflecting or scattering light is not determined by the property of the material itself, but is determined by the method of forming the layer. Therefore, when the material is to be used for a display devices, both light-transmission type and reflection type display device may be formed.
One of the fundamental modes of construction of the element which exhibits the electrochromic phenomenon is disclosed in U.S. Pat. No. 3,521,941 (Deb et al.). The element disclosed in this patent has a current carrier permeable insulator is laminated on an electrochromic layer using a transition metal compound such as WO.sub.3, MoO.sub.3 and the like and the laminate is interposed between a pair of electrodes. In the element of this patent, a resin film, for example, polyester, CaF.sub.2, SiO.sub.2, MgF.sub.2 and the like is used as the current carrier permeable insulator. However, these materials are often not satisfactory for practical use in view of the inferiority in the electrochromic efficiency, the displayed color density and the response time. In these points, there still exist room for further improvement in the electrochromic device. The term "electrochromic efficiency" as used herein means a proportional constant between the variation in color density and the quantity of injected charge since the variation in the color density in this element is determined by the quantity of the charge injected to this element (IEEE Transaction On Electron Devices, Vol. ED-22, No. 9, 1975, p. 749, "Performance characteristics of electrochromic displays," Ifay F. Chang and Webster E. Howard).